1. Field of the Invention
This invention relates to a lens system for projection of an enlargement of an image appearing on a cathode ray tube onto a projection screen.
2. Related Art
U.S. Pats. Nos. 4,300,817, l4,348,081 and 4,526,442 to Betensky all disclose a lens system which employs lenses having aspheric surfaces for projection of an image formed on a cathode-ray tube (CRT) onto a projection screen. This lens system produces an enlarged image of high brightness. However, the length of the beam path from the face glass of the CRT to the screen in a projection system using the lens system of Betensky is too long to be suitable for a compact projection set. That is, the Betensky system provides insufficient enlargement.
In order to increase the enlargement and to shorten the length of the beam path compared to the Betensky design, a lens system has been constructed which comprises an aspherical lens of relatively weak power near the image side (i.e. toward the screen), a biconvex lens providing most of the enlargement of the lens system, an aspheric convex lens of relatively weak power, a concave lens of relatively large negative power, and a CRT with a face glass having an aspheric surface on which the phosphor elements are mounted. The aspheric surface is of minimum diameter on the optical axis. This lens system is disclosed in copending U.S. patent application, Ser. No. 911,070, filed Sept. 24, 1986, (the "'070 application") incorporated herein by reference.
The lens system described in the copending '070 application provides a relatively short distance, approximately 700 mm, between the screen and the lens. The inventors have found that the lens system disclosed in the '070 application tends to produce a phantom image, which seems to be caused by a multiple reflection of the light beam from the phosphors of the CRT, particularly in the brighter portions of the image. Since the brightness of the phantom image is about 1.3.times.10.sup.-3 of the brightness of the image itself, the phantom image is not normally visually distracting. However, the phantom image caused in response to a white vertical bar in the image can be distracting, especially when the remainder of the image and the room in which the television is located are relatively dark.